Blizzard Entertainment is reportedly working on a free-to-play project, though the exact identity of the game is unknown. This is being referred to as the first step in an overall shift in the company's business strategy, ultimately leading to offering both subscription and microtransaction-based titles side-by-side.
The Develop source claims that the F2P title is an unannounced game, which would discount the most obvious possibility of Blizzard DOTA. Project Titan technically hasn't been formally announced, so that could be the mystery game, or it could be an entirely new project.
We've seen speculation about Titan as a free-to-play game before, especially with the possibility of alternate advertising models. Meanwhile Blizzard's current hit MMO, World of Warcraft, is free-to-play up to level 20.
Blizzard COO Paul Sams noted last year that a F2P model is "possible" for the company, but "at this point we haven't decided to make a game where that is the model. But that doesn't mean that we won't." Titan was in development at the time, so if the game is going free-to-play, it must have been decided since then.